by Phil Salmon
How a small group of people from Africa changed the world.
Paleoclimatology, the study of climate in the past all the way back to deep time, is one of the pillars of climate science. It shows us the wide range of climates that have existed on earth, from near-global glaciation in “snowball-earth” epochs to hothouses where the poles were forested. It puts in a correct perspective discussion about climate change – dismissing any notion that climate has ever done anything other than endlessly change. And it provides an alternative to computer simulated models of climate, in which, given enough knowledge of conditions and influences on climate at past times, virtual experiments can be run by fast forwarding from chosen time points, analyzing the climate record to disentangle cause from effect.
The trouble is, of course, that conditions on earth and influences on climate are poorly known in the distant past, and this knowledge deteriorates in quality with further rewinding to earlier and earlier times.
However in the most recent ten million years or so, the paleoclimate record is good enough for us to know quite precisely how important tectonic changes have occurred, such as the portentous joining of the two America’s, the formation of the Mediterranean Sea separating Africa from Europe, and the opening of a Rift Valley, Africa’s incipient tearing away from Eurasia to become in the future a colossal island on its own.
And within the last million years our knowledge is still further improved by the availability of ice core data showing us the detailed ups and downs of climate temperature (and, following obediently behind it, CO2) during the Quaternary period of alternating glacial and interglacial intervals.
Alongside climate science, another scientific field shares a focused interest in this recent timescale of a few million years: that of human evolution. It turns out that the two subjects are intimately linked. Not only did humans (self-evidently) survive the violent extremes of climate over the course of the glacial Quaternary period. It is considered by evolutionary biologists likely that the acute pressure on hominids to adapt or die as their habitats alternated between forest, grasslands and deserts, and as they periodically endured violent excursions of climate change, led to the run-away expansion of the human brain and our eventual attainment of consciousness accompanied by complex symbolic language and our imaginative, introspective inner world. And stemming from this, our farming, building of tools and machines led to the transformation of the earth’s surface. The physiological investment in a grotesquely bulbous head and huge brain consuming 25% of all our oxygen and energy, necessitated sacrifices in muscle mass, speed and strength. This increased our ancestors’ vulnerability to predation, leaving them even more in need of intelligent life strategies – thus the growth of mind ran along a positive feedback for a while.
Yuval Noah Harari’s recent book “Sapiens” focuses on a mysterious aspect of our evolution – namely that although our large brains and intelligence (complete with social, cultural and language abilities) have existed for at least 70,000 years, our agricultural, technical and intellectual achievements leading to our current industrialized lifestyle and landscape, have happened only in the last few centuries – relatively speaking in the blink of an eye. Why did this explosive advance wait for tens of thousands of years, when humans would have been capable of all that we do today, 70,000 years ago? (Apart, maybe, from “living on milk and alcohol”.) To date there is no clear answer to this question. In this article we will look at the possibility that climate might be at least partly responsible.
Climate change almost certainly played an important role in human evolution – as expounded for instance in John and Mary Gribbin’s book “Children of the Ice” (see references at the end.) And as we shall see below, the climate of prior glacial periods was violent beyond anything we have experienced on our mild and molly-coddled Holocene, possibly at times threatening our very existence. But paradoxically, at the same time, making us what we are.
Human evolution and the out-of-Africa migration
The last common ancestor of humans and chimpanzees lived 7 million years ago. Since then, contrary to superficial misconceptions of a single succession of ancestors leading to modern humans, an evolutionary tree grew out many branches of different human types, some which left descendants, others only genes.
Africa was the origin of the very first hominids and humans, several million years ago. (Our common ancestor with chimpanzees may even have been a bipedal hominid.) From Africa our ancestors radiated (spread out) into the rest of the world, at various times. Archaic humans of the species Homo erectus and Homo heidelbergiensis migrated out of Africa as long as 2 million years ago and spread widely throughout Eurasia and Europe. These early migrants left ancient descendants in Europe called the Neanderthals, and in Eurasia called the Denisovans. One remarkable group of even older humans (Australopithecines) made it as far as the Indonesian island of Flores, where a million years of isolation and island dwarfism led them to become so much smaller than other humans – only half the height – that scientists discovering their diminutive skeletons called them “hobbits” (to the legal indignation of Peter Jackson’s “Lord of the Rings” entourage of lawyers). This unique hominid Homo floresiensis survived on Flores until fifty thousand years ago – the time that we modern humans showed up. Whereupon the hobbits promptly died out.
However the more recent migration of modern Homo sapiens which took place 60,000 years ago had special significance. It was the first departure of Homo sapiens from Africa after they had become “behaviorally modern”, acquired skills of advanced tool use and language, and had begun creating art works. It should be noted that at this time there were only African modern humans on earth: today’s racial diversity of modern humans would come much, much later. For instance the pale skin of northern Europeans lay in a still distant future, less than 10,000 years ago in the Holocene interglacial.
The study of genetics compliments the discoveries of skeletal remains by paleontologists. The strings of DNA present in all the cells in our bodies are like mini-barcodes packed with a huge amount of information about ourselves and our ancestors. Genetic analysis shows that a single migration event took place about 60,000 years ago, by a small group of people genetically marked with “mitochondrial haplotype L3”. Astonishing, almost all humans on earth today, with the exception of some in Africa, are descended from this single exodus. This founding group of migrant ancestors consisted of just a few hundred individuals, the number of people at a typical wedding party. They departed Africa via the Red Sea and the Sinai mountains, foreshadowing the biblical Exodus. This small group of modern L3 labelled humans spread from Africa all over the world and are the ancestors of all humans outside Africa today.
(Please note that the fact that there were numerous “exodi” from Africa both before and after the 60 kya event, does not alter or contradict the clear evidence from genetics, from both maternal mitochondrial and male Y-chromosome analysis, that most humans today are indeed descended from that single 60 kya breakout. As Richard Dawkins showed mathematically in “The Ancestor’s Tale”, prior to 20,000 years ago, everyone alive was ancestor either to everyone living today, or to no-one.)
There was a dark side, however, to this sudden radiation of us moderns. It directly coincided with the disappearance of the Neanderthals, Denisovans, “hobbits” and other descendants of the much earlier archaic human radiations from Africa. This is circumstantial evidence of a kind of genocide – not our finest hour. Although the Neanderthals and other archaic humans did interbreed to a limited extent with moderns, such that today as much as 4% of our genome is of Neanderthal descent (if we are Caucasian) or Denisovan (if we are Asian) or from H. floresiensis hobbits (if we are from New Zealand).
A recent genetic study of present day people groups in Africa (Hammer et al. 2011) found that there had been interbreeding between modern humans and archaic remnants of Homo erectus in Africa as recently as 35,000 years ago.
Figure 1. A reconstruction of the face of a female ancient human of the species Homo erectus or Homo heidelbergiensis, made by the Smithsonian Museum of Natural History in the USA.
Ice ages and past climate chaos
The climate of our earth is chaotic and always changing, with or without human help. This has been especially true in the last interval of slightly less than three million years, called the Quaternary Period. During this period the world has moved into glaciation, which means the appearance of large iced up areas at both the north and south poles.
The Quaternary period, which lasts until today, is the time frame over which humans have appeared. Technically speaking we are in ice age right now. This is where talking about being in an ice age gets complicated. For throughout all of the Quaternary so far, the earth has oscillated or “flip-flopped” between cold glacial ice age conditions and shorter interludes of warmer climate called “interglacials”. We are in such an interglacial right now – we call it the Holocene. Such warm intervals have typically lasted about 10,000-15,000 years; our own Holocene interglacial is just over 12,000 years old. This sounds like a long time – indeed practically all of recorded human history has taken place within the Holocene. However the frigid glacial periods which alternate with the interglacials are much longer, between 40,000 and 100,000 years in duration. Thus most of the Quaternary so far has been in a state of ice age.
The timing of the glacial cycles over the Quaternary has had a certain rhythm and regularity, and this has not been by chance. Helping the climate system to roll on a regular basis between glacial and interglacial attractors are periodic changes in the earth’s spin and orbit round the sun, called the Milankovitch cycles after the Serbian mathematician who worked out this connection. (This had in fact been first realized by a Scotsman, James Croll, but it was Milutin Milankovitch who fully established how regular wobbles in our orbit translate into the timing of the ice ages). There are three Milankovitch cycles with different periods: eccentricity (100,000 years), obliquity (41,000 years) and precession (19-22,000 years). The glacial-interglacial cycle is principally driven by obliquity, constrained by eccentricity and precession, with a 6,500 year lag – the time it takes the sun to warm up the oceans.
Figure 2. Of the three Milankovitch cycles, obliquity plays the primary role in the timing of the glacial cycle during the late Quaternary. Every single interglacial coincides closely with a peak of obliquity lagged by 6500 years (thanks to Javier for this figure!). But not every (lagged) obliquity peaks produced an interglacial – only those that coincided with the right conditions of eccentricity, the modulation of precession and consequent 65 degree N summer insolation. 6500 years is the time taken for Milankovitch insolation increase to warm the deep oceans.
This behavior is in fact quite typical of a chaotic system such as the earth’s climate, flip-flopping between two “quasi-stable” states, glacial and interglacial, unable to “make up its mind” which one it prefers. By the way the few stable states between which such a chaotic system will jump, have a name – they are called “attractors”. Chaos theorists talk about a “probabilistic landscape”, in which attractors are the valleys of probability separated by improbable mountains but sometimes connected by a pass or “saddle” which allows the system state to roll – sometimes with a bit of help from outside – from one valley to another.
The chaotic dynamics of ice ages get still more complicated. Within the long glacial periods there is a lot of climate instability and very sharp episodes of climate change, far more dramatic than anything seen in recent decades. Although glacial periods are longer than the warm interglacials, paradoxically climate is intrinsically less stable during the glacial intervals since the bigger temperature gradient between equator and poles lends more energy to the ocean-atmosphere system as a whole. While today’s politicians and scientists do their best to avoid the hazards of one or two degrees warming over a century or so, during recent glacial intervals temperature changes many times larger, of 10 degrees or more, took place regularly over similar short timescales. Graphs showing frozen records of the temperature of the Northern Hemisphere from ice and sediment cores show this clearly over the last hundred thousand years. This represents the time interval between our Holocene and the previous interglacial called the “Eemian”. In this glacial period there are many sharp upward spikes, or “micro-interglacials” in which temperatures rose – then fell – by up to ten degrees within only a century or a few hundred years. The technical term for these excursions is Dansgaard-Oeschger (D-O) events. These would have been very traumatic times to live through.
Figure 3. The dO18/O16 isotope ratio indicating the violent swings of northern hemisphere climate temperature during the last glacial period between the Eemian and the Holocene. Fluctuations with amplitude of almost 10 degrees C over just a few centuries or millennia occurred many times during this glacial interval (Dansgaard-Oeschger events).
What did all this mean for human populations living during the last glacial period? Of course it had profound implications. It meant that every few centuries or millennia, their lives were heavily disrupted by rapid and severe episodes of climate change, either warming or cooling – or both in succession. Such major changes would be linked to shifts in atmospheric and ocean circulation that would in turn affect food availability for human populations trying to build a way of living. It might have been these traumatic swings of temperature and ecological stress associated with them, that prevented human civilization from taking root over a long period of millennia. This could answer the question of Yuval Harari, of why human technical and cultural advancement only really gathered pace within the last part of the Holocene.
Yes there is a role for CO2
The odyssey from Africa took place about 60,000 years ago. From graphs of temperature over the last glacial cycle, it can be seen that this was not quite the coldest part of the glacial period. That lay ahead at around 20,000 years ago, the “glacial maximum” during which the climate also became very dry, with high winds blowing dust storms from arid plains. Ice core records show that, at that time, the air even at the poles carried clouds of dust from vast and windy deserts. One possible reason for the dust and aridity in mid and high latitudes during the last glacial maximum was the fall of carbon dioxide (CO2) concentration in the atmosphere to low levels of less than 200 ppm (parts per million) – levels at which CO2 “starvation” begins to cause stress to plants, and which cause ecosystems to change from forest to grasses and grasses to desert. For instance at less than 200ppm CO2, tree regrowth after fires can become too slow for new trees to be big enough to resist the next fire, so trees are replaced by grass (Bond et al. 2003). And the 1930’s dustbowl years in the USA show how tree loss can lead to soil loss, dust and aridity. Robert Ellison has advanced a hypothesis that this aridity at the glacial maxima, accentuated by CO2 starvation, formed a component of a feedback loop that decreased albedo on dust-covered ice sheets and brought about the abrupt termination of each glacial maximum. Clearly our human ancestors would have suffered severe perturbation to their way of life during the cold, arid and dusty glacial maxima.
Recent research by Jessica Tierney of the University of Arizona, USA, reconstructed climate at the “Horn of Africa” – the region including Ethiopia and Somalia at the south of the Red Sea – from cores of sediment from the Arabian Sea. Tierney’s team looked for chemical traces of the wax from leaves blown out to sea in the sea-floor sediments, allowing them to reconstruct which periods over the last 200,000 years were either arid and dry or green and wet. They discovered – to their surprise – that at the period around 60,000 years ago, when the exodus of modern humans from Africa took place, the Horn of Africa had turned arid and dry. This is puzzling – it would have made our Odyssey from Africa more challenging – but evidently it didn’t stop it from happening.
It is considered by scientists that this staccato pattern of repeated sharp climate changes was a key factor in the evolution of large brains and intelligence that resulted in the emergence of us humans. With repeated alternation of large parts of Africa between forest, savannah and desert, our ancestors would have needed to be smart and to anticipate and make contingencies for the future, and to quickly innovate to exploit new unexpected circumstances. Without doubt they “lived in interesting times”!
Ice age animals that are now extinct.
At that time around 60,000 years ago, many creatures lived on earth which are now no longer with us. This includes Eurasian populations of straight-tusked elephants, hippos, rhinos and cave bears. Further north nearer the glacial fronts, woolly mammoths, woolly rhinos and the musk ox roamed the frosty plains. Now extinct predators that still lived at that time included sabertoothed cats, and also cave lions and a European leopard. And on Madagascar the great giant elephant bird Aepyornis – a ratite like ostriches but way, way bigger, was king of all it surveyed.Figure 4. The skull of a sabertooth cat, species Smilodon, and an artistic reconstruction. Note the large bony crest at the rear of the skull (left) for powerful jaw muscle attachment. I don’t think that this reconstruction (right) does justice to how terrifying a creature Smilodon was.
Over the Atlantic, 3 million years ago at the start of the Quaternary, North and South America had only just joined together at Panama, leading to a convulsion of migration and mixing between the two hitherto separate continents. Many great megafauna lived then in the Western Hemisphere that have since disappeared: giant sloths, the American lion, a giant tortoise, sabertooth cats, camels, the “dire wolf”, the mastodon and the armadillo-like glyptotherium.
Where did they all go? What caused so many wonderful big creatures to die out? Scientists speak of two principal causes, with ongoing debate as to their relative importance; these are, stress from changing climate, and hunting by us humans. A human hand is evident in some extinctions. In North America especially, creatures that had never been exposed to modern human hunters suddenly confronted them 12,000 years ago when melting ice (and still decreased sea level) allowed modern humans to cross for the first time into the American continent from Siberia and East Russia. The American megafauna’s naivety to humans led to their demise. By contrast, African megafauna – our elephants, hippos, lions, giraffes and so on – had experienced the company of us humans for millions of years throughout our evolution. This enabled them to adapt to survive alongside us; at least so far.
However before we get too judgmental of our rapacious ancestors, it seems that opinion is moving in the direction of climatic stress as the culprit for most megafaunal extinctions. The severe cold of the last glacial maximum combined with rapid climate fluctuations, the “micro-interglacials” (D-O events) discussed above, traumatized ecosystems and contributed to the extinctions of these large creatures.
Mount Toba and the great bottleneck
One baleful climatic player in the ancient history of both humans and other animals needs to be mentioned. Mount Toba was an island in Indonesia which exploded in a devastating volcano – what scientists call a supervolcano – about 75,000 years ago. A supervolcano is thousands of times bigger than ordinary volcanos, able to disrupt climate globally, and Mount Toba is estimated to have erupted 3,000 cubic km of rock as pulverized ash, covering much of South Asia in a 6 inch (15cm) deep ash layer. This unimaginably violent eruption is many times bigger than anything experienced in recorded history. (By contrast the Mount St Helens volcanic eruption in Washington state, USA in May 1980 ejected less than 3 cubic km of ash.) Up to ten years of global winter followed the Mount Toba supervolcano. Genetic analysis suggests that the worldwide human population was decimated by Mount Toba to a mere 1000 breeding pairs – the population of just one small town or village today. Some other animals also show evidence of the “genetic bottleneck” of a similar drastic decline in population, such as orangutans and tigers in Asia and chimpanzees and cheetahs in Africa. Eventually humans, animals and ecosystems recovered from this catastrophe. It happened a few thousand years before the time of the 60 kya odyssey, and partly explains why on the earth at that time, “few in number were the people”.
(In fact the reason that the 60,000 year ago breakout group from Africa became ancestral to all the world’s population outside Africa now, might be partly good luck: they were the first exodus from Africa after Mount Toba decimated worldwide populations descended from earlier breakouts of modern humans, of which there is evidence as far back as 120,000 and even 200,000 years.)
Could mount Toba even have been the near-fatal stressor that pushed humans into that final expansion of brain and mind, in a desperate fight for survival, that led to the “great leap forward” to modernity, language, culture and cave art, 70,000 years ago?
Earth’s violent geological history did not quite kill us, but helped to make us what we are – creatures with the intelligence to discern good from evil, and the freedom to choose one or the other.
A narrative poem about the 60 kya breakout
Although much has been written about the paleontology and evolutionary biology of developing humans, there is very little fiction or poetry written about the majority of human history, the 200,000 years that anatomically modern – and the 70,000 years that behaviorally modern – humans have lived on earth. There has been little expression of curiosity in the form of story-telling about most of this period. Our focus has been exclusively on the tail end of the Holocene, with an echoing silence from most of our long history before that.
In an amateur attempt to put this right, I wrote up as a narrative poem a run-on story that I told to my daughters some years ago when they were of that age where they demanded a bedtime story every evening. My three girls provided many of the illustrations for the book I eventually wrote. Although the story is lighthearted in character, it makes some important points about our prehistory and exodus from Africa. Climate, for instance, is an important player, a threat in the background that drives the migration event.
The result of this effort is “Odyssey from Africa (and the Adventures of Ipiki)”, a story about a small group of people who inadvertently changed the world, sixty thousand years ago. Fleeing a village witch-hunt, a fisherman with his wife and twin children embark on a flight for survival, that grows into a journey of discovery and inspiration. Joined along the way by companions both animal and human, the scope of their odyssey expands into something far greater than they would ever have imagined.
“…And our world is so much larger
Than the limits of our vision
What to us seem wide horizons
Are a spot upon the surface
Of a world immense and wondrous
Filled with things beyond our knowledge”
It’s written in the same inverse iambic pentameter meter of the poem “Hiawatha” about a legendary native American hero, by Henry W. Longfellow; it’s also about the same length.
At the Amazon page for this book (available in paperback and Kindle, link below), you can read the first few chapters by clicking on “preview” or “look inside”.
Here are the first seven verses:
Mvuvi (The Fisherman)
At the dawn of humankind in
Africa’s great land of sunlight
This adventure of survival
Of a family’s arduous journey
One of exile and betrayal
And of hopeful brave endurance
Happened long before all memory
Only scattered bones bear witness
Mighty ice-sheets of the Arctic
Would again repeat their south-march
Hold the world in age-long winter
Then retreat to warming sunshine
All between this ancient voyage
And the present time of writing
Days of our long distant fathers
Few in number were the people
Somewhere on the south-east seaboard
Was a modest fishing village
Where a hundred families prized their
Living from the restless ocean
Here lived Han with his wife Kwona
Their son Matto, daughter Lisa
Twins of near a dozen summers
Old enough to help their parents
Lisa watched her mother’s fingers
Make, like magic, food and clothing
Matto learned beside his father
As a fisherman’s apprentice
References [References]
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